Bloom's syndrome cells are characterized by abnormally high spontaneous mutation rates. It has been reported that Bloom's syndrome cells are hypermutable with spontaneous mutation frequencies five- to ten-fold higher than normal cells. Bloom's syndrome cells are singularly susceptible to transformation by DNA transfection. Individuals with Bloom's syndrome have an increased rate of neoplasia, infertility, immune deficiency and infection.
Bloom's syndrome is characterized by a high level of chromosomal aberration, particularly sister chromatid exchanges. It is, along with Fanconi's anemia, and ataxia telangiectasia, one of three genetic diseases in which unrepaired chromosomal breakage occurs with several times the frequency observed in normal individuals.
Bloom's syndrome is inherited as an autosomal recessive condition. The Bloom's syndrome gene appears to be widespread in the population, with cases reported in Ashkenazi-Jewish, Japanese, American Black, Western European Christian, and non-Jewish semitic (Mohammedan) ethnic groups.
In addition to unrepaired chromosome breakage, other cytogenetic abnormalities are present with an abnormally high frequency, such as chromosomal rearrangements, deletions and "fusion fissures". The latter appears to result from a break in a single chromatid in each of two chromosomes.
Clinically, Bloom's syndrome is associated with congenital malformations and a predisposition to malignancy. The disease is characterized by low birth weight, not due to prematurity, and severe, generalized growth retardation. Bloom's syndrome presents a telangiectatic erythema which primarily affects facial areas, and a sun-sensitivity which can initiate or accentuate the facial lesions.
There are no methods for specific diagnosis of Bloom's syndrome at the cellular, molecular or biochemical level. The sole means for diagnosis is a clinical evaluation by a physician as symptoms became apparent. However, even in areas where excellent health care is available, diagnosis may be difficult due to the diversity of symptoms and the degree of their severity. Diagnosis of Bloom's syndrome may be impeded where health care is unavailable.
What is needed is a simple, reliable biochemical/immunological test for diagnosing Bloom's syndrome. In particular, there is a need to identify children afflicted with Bloom's syndrome early after birth but prior to appearance of clinical symptoms, and in utero at an early stage of gestation. Since Bloom's syndrome patients are cancer-prone, diagnosis would permit careful monitoring for detection of tumors at an early stage when the cancer is most responsive to treatment.
Recent studies have demonstrated that eucaryotic cells actively regulate DNA repair pathways during the defined temporal pattern of gene expression observed during cell proliferation. In comparison to quiescent cells, proliferating cells exhibit increased levels of DNA repair enzyme, increased excision repair synthesis, and faster removal of DNA lesions. In serum-synchronized normal human cells, DNA repair pathways are enhanced prior to DNA synthesis and are decreased during S phase.
No difference has been observed in the excision of DNA adducts, or in the level of DNA repair enzymes, in non-growing Bloom's syndrome cells. However, in spontaneously hypermutable Bloom's syndrome cells, there is a failure to enhance DNA repair pathways prior to DNA replication. Instead, both nucleotide excision repair synthesis and base excision repair synthesis are enhanced coordinate with DNA replication.
The most important DNA repair pathways in humans depend on the excision of an altered residue or group. The group of enzymes known as DNA glycosylases catalyze the cleavage of base-sugar bonds in DNA. They act only on altered or damaged nucleotide residues.
Uracil DNA glycosylase functions as an initial enzyme in the base excision repair pathway to remove uracil residues from DNA by cleavage of the base-sugar glycosyl linkage producing an apyrimidinic site in DNA. Uracil may arise in DNA through the mutagenic deamination of cytidine, or through incorporation of deoxyuridine 5'-monophosphate during DNA replication. Human uracil DNA glycosylase has been extensively purified and characterized. It has a molecular weight of about 37,000 daltons. Uracil DNA glycosylase may be measured by quantitation of in vitro enzymatic activity or by enzymelinked immunosorbent assay ("ELISA") with anti-human-uracil DNA glycosylase monoclonal antibodies. Arenaz, P. and M. A. Sirover, Proc. Natl. Acad. Sci. USA, 80: 5822-5826 (1983).
Hereinafter, "UDG" shall mean the human base excision-repair enzyme uracil DNA glycosylase.
"Bloom's syndrome UDG" shall mean any genetic variant of UDG characteristic of Bloom's syndrome.
"Anti-human UDG monoclonal antibody" shall mean a monoclonal antibody which recognizes normal human UDG.